This invention relates to electronic components for implantable medical devices, and more particularly to batteries for implantable cardioverter/defibrillators.
Implantable Cardioverter Defibrillators (ICDs) are implanted in patients susceptible to cardiac tachyarrhythmias including atrial and ventricular tachycardias and atrial and ventricular fibrillation. Such devices typically provide cardioversion or defibrillation by delivering low voltage pacing pulses or high voltage shocks to the patient""s heart, typically about 500-800V. The ICD operates by detecting a fast heart rate or tachyarrhythmia, upon which a battery within the device housing is coupled via an inverter to a high voltage capacitor or capacitor pair to charge the capacitors. When the capacitor reaches a desired voltage, charging is stopped and the capacitors are discharged under control of a microprocessor to provide a therapeutic shock to the patient""s heart.
While transcutaneous rechargeable battery systems have been contemplated, for example as provided in U.S. Pat. No. 5,991,665 to Wang et al., such a system has never been implemented in an ICD because of the lack of an acceptable battery recharging system. Therefore, it is generally expected that the battery must store all the energy needed for continuous monitoring and analysis of sensed electrogram and other physiologic signals, for telemetric communications and for potentially numerous shocks over the life of the device, and must retain the energy with minimal leakage to provide a long xe2x80x9cshelf lifexe2x80x9d of at least several years, even if not frequently employed for shocks during its life. Thus, the energy storage capacity of the battery is important.
In addition, a battery must be capable of high current rates needed to charge the high voltage capacitors in a short time, so that a therapeutic shock may be delivered within a short time interval after the device has detected and diagnosed a need for the shock. If the battery has an excessive internal resistance, the current flow rate will be limited, delaying capacitor charging. This may result in syncope, ischemia (oxygen starvation) of critical organs and tissues. As a general principle, the sooner the therapy can be delivered following a detected episode, the better prospects are for the patient""s health. In addition, it is believed that therapy delivered more promptly requires a lower energy therapy, allowing the conservation of the battery""s energy to extend the device life before replacement is required.
Also, an omnipresent concern with implantable devices is device volume. A small device permits more flexibility in implant location, and provides improved patient comfort. There is generally a trade-off between size and storage capacity, with larger batteries providing more capacity. To mitigate this trade-off, batteries with high energy density (watt-hours per unit volume) are desired.
However, there is a trade-off between energy density and the current flow rate discussed above. The highest density cells, such as Mercury-zinc and Silver-zinc types are suited to applications where a moderate current draw occurs, but these have a high internal resistance that prevents them from providing the high current flow rate needed for rapid capacitor charging.
Thus, ICD designers have adopted low internal resistance battery chemistries such as Lithium Silver Vanadium Oxide (SVO), using one or more such cells. These provide the required rapid capacitor charging, but at the cost of somewhat compromised energy density. In addition, SVO and comparable performance batteries are expensive compared to other battery chemistries that lack only the needed current output. Also, over the life of existing devices, as SVO battery voltage diminishes, the time interval between diagnosis of an arrhythmia and completion of capacitor charging increases, so that the effective device life is limited due to the concerns noted above about delayed treatment.
In the past, certain implantable defibrillators were designed to reduce the demand on the battery used for critical, high current charging duties by employing a separate second cell having higher energy density and lower current capacity. This high density cell serves device circuitry not requiring high current rates, reducing the depletion of the lower energy density cell devoted to capacitor charging. While this may permit a slightly extended life, or slightly reduced size, the benefits are limited, because the low current battery circuitry adds size, complexity, and introduces a parasitic current load that will tend to reduce longevity.
The disclosed embodiment overcomes the limitations of the prior art by providing a battery network for an implantable cardiac stimulation device having first and second batteries with different battery chemistries connected in parallel with each other. The first battery has a higher resistance and a greater energy density than the second battery, and the second battery has greater current carrying capability than the first battery. Defibrillator circuitry is connected to the batteries. In a preferred embodiment, the first battery has a higher voltage than the second battery, and the second battery has a lithium anode. The device operates by detecting a need for treatment, providing current flow primarily from the second battery to the defibrillator circuitry, then providing current flow from the first battery to the second battery.